Rectifier and voltage divider means utilized as by-pass means for converters



Jan. 3i, w? c. l. BOKSJG ETAL 3,302,094

RECTIFIER AND VOLTAGE DIVIDER MEANS UTILIZED AS BY-PASS MEANS FOR CONVERTERS Filed May 5, 1964 BY K 70% United States Patent M 3,302,094 RECTIFIER AND VOLTAGE DIVIDER MEANS UTILIZED AS BY-PASS MEANS FOR CON- VERTERS Carl Ingvar Boksjii, Ludvika, and Erik Gunnar Hedenbo, Smedjebacken, Sweden, assignors to Allmanna Svenska Elektriska Aktiebolaget, Vasteras, Sweden, a corporation of Sweden Filed May 5, 1964, Ser. No. 364,979 Claims priority, application Sweden, June 7, 1963, 6,294/ 63 5 Claims. (Cl. 321-27) The present invention relates to a voltage divider for a by-pass rectifier in a converter.

In converter stations having several constituent converters it is usual for the individual constituent converters to be provided each with its own by-pass rectifier and short-circuiting means, for example in the form of an isolator. The by-pass rectifier is usually similar in design to the normal operating rectifier and, like these is provided with a number of intermediate electrodes connected to a voltage divider inserted between the anode and cathode of the rectifier. The voltage divider usually consists of a number of capacitors and resistors connected in series and/or parallel. The purpose of the voltage divider is to obtain a suitable voltage distribution over the whole mercury arc rectifier, especially during the ignition process. Normally such voltage dividers to a great ex tent provide a linear distribution of voltage between anode and cathode which, particularly for the operating rectifiers, is what is desired since the grid control usually gives them the required high ignition voltage. However, if it is a question of a bypass rectifier which has been short-circuited by means of its short-circuiting device, the voltage available for the ignition is rather low, i.e. only corresponding to the arc voltage drop in the short-circuiting device while it is being opened. In such a case it has been found difiicult to obtain sufficient voltage across the lower part of the rectifiers, i.e. in the discharging space between the cathode and the lowest intermediate electrodes. The voltage divider might for this reason be made with higher impedance, for example in the form of higher resistance in that part of the voltage divider which lies between the cathode and the nearest electrodes. Since the arc voltage in the shortcircuiting means increases only rather slowly in comparison with the time constants of a capacitive voltage divider, such an unequal distribution in a resistive voltage divider parallel-connected to the capactive voltage divider would give the desired result during ignition of the rectifier. However, if the unequal resistance distribution is to have any influence when low voltage prevails over the rectifier, it must be rather pronounced, which impairs the properties of the rectifier in other respects.

Instead, it is proposed according to the invention to connect in the part of the voltage divider lying nearest the cathode a device to increase the voltage across this part and this voltage increase should be of the same order of magnitude as the arc voltage drop arising when the short-circuiting means is opened. However, the voltage increase should be low in relation to the voltage prevailing over the constituent converter during normal operation.

Such a device may, for example, consist of a voltage dependent resistor or zener-diode. According to another embodiment of the invention said device may consist of a voltage source connected in the voltage divider. Voltage dividers of the type mentioned are often composed of two parallel branches one of which is preferably resistive while the other is preferably capacitive, which Patented Jan. 31, 1967 two branches are joined by a number of cross-connections. In this case said device is suitably inserted in the lower part of the resistive branch. The last-mentioned embodiment means that the capacitive part of the voltage divider is charged the whole time and the rectifier is thus permanently in readiness for ignition and can be ignited with very low anode voltages. Such a connected voltage may suitably be of the order of magnitude of a few hundred volts.

The invention will be further described with reference to the accompanying drawing where FIGURE 1 shows a converter station while FIGURES 2 and 3 show by-pass rectifiers with different voltage dividers according to the invention.

In FIGURE 1 is shown a converter station 5 inserted between a DC. conductor and earth. The A.C. side of the station is not shown. The station comprises three constituent current converters 6, 7, 8, each provided with a by-pass rectifier 1 and short-circuiting means 4. The by-pass rectifier 1 is provided with a voltage divider 2, 3.

The by-pass rectifier 1, the voltage divider 2, 3 and the short-circuiting means 4 are further shown in FIG- URES 2 and 3. These figures show a mercury arc rectifier 1 with cathode 11, anode 13 and a number of intermediate electrodes 12 which are connected by seriesresistors 23 to a voltage divider comprising two parallel branches 2 and 3. Of these two branches branch 2 is preferably capacitive containing a certain resistance 21 and a number of capacitors 22. However, branch 3 is preferably resistive and is composed of a number of resistors 31. The two branches are joined by a number of cross-connections. In FIGURE 2 a voltage-dependent resistor 32 is connected in the lower part of branch 3 of the voltage divider. When the rectifier is to be ignited after having been short-circuited, the accessible ignition voltage will be relatively low, i.e. only corresponding to the arc voltage in the short-circuiting means when this is opened, and this relatively low voltage will be substantially across the resistor 32 and thus also across the lower part of the rectifier, i.e. between the cathode and the lowest intermediate electrodes. If, however, it is a question of igniting the by-pass rectifier under high voltage, which may be the case when a constituent converter is to be by-pass connected and blocked, the resistance in the voltage dependent resistor 32 will be relatively low and the rectifier voltage will therefore be distributed to a great extent linearly over the voltage divider, as is desirable for ignition under high voltage. Instead of a voltage dependent resistor as element 32 a zener-diode may be used having a zener-voltage of -200 volts.

A third possibility is shown in FIGURE 3 where the element 32 consists of a rectifier bridge fed from an A.C. source 34 by a transformer 33. The direct voltage obtained is suitably of the order of magnitude of 100 volts and this embodiment has the advantage, with the type of voltage divider shown having one capacitive and one resistive branch, that the capacitors in the capacitive branch will be permanently charged, thus making it possible for the rectifier to be ignited with very low anode voltages.

We claim:

1. In a converter station comprising a plurality of constituent current converters; each constituent converter being provided with by-pass means; said by-pass means comprising a by-pass rectifier having an anode, a cathode, and intermediate electrodes, said by-pass means further including short-circuiting means; a voltage divider connected between the anodeand cathode-conductors of said by-pass rectifier, said intermediate electrodes being connected to said voltage divider; means connected in said voltage divider near the cathode of said by-pass rectifier responsive to opening of said short-circuiting means to increase the voltage between the cathode and the adjacent intermediate electrode; said increased voltage being of the sameorder of magnitude as the voltage across an arc arising upon opening of said short circuit means; but being substantially less than the normal voltage over the corresponding constituent converter.

2. In a converter station as claimed in claim 1; said means for increasing the voltage comprising a voltage dependent resistor.

3. In a converter station as claimed in claim 1; said means for increasing the voltage comprising a zenerdiode.

4. In a converter station as claimed in claim 1; said means for increasing the voltage comprising a DC. voltage source.

5. In a converter station as claimed in claim 1; said voltage divider comprising two parallel branches; one of said branches being resistive; the other of said branches being capacitive; said means for increasing the voltage being connected in said resistive branch.

No references cited.

JOHN F. COUCH, Primary Examiner.

G. GOLDBERG, Assistant Examiner. 

1. IN A CONVERTER STATION COMPRISING A PLURALITY OF CONSTITUENT CURRENT CONVERTERS; EACH CONSTITUENT CONVERTER BEING PROVIDED WITH BY-PASS MEANS; SAID BY-PASS MEANS COMPRISING A BY-PASS RECTIFIER HAVING AN ANODE, A CATHODE, AND INTERMEDIATE ELECTRODES, SAID BY-PASS MEANS FURTHER INCLUDING SHORT-CIRCUITING MEANS; A VOLTAGE DIVIDER CONNECTED BETWEEN THE ANODE- AND CATHODE-CONDUCTORS OF SAID BY-PASS RECTIFIER, SAID INTERMEDIATE ELECTRODES BEING CONNECTED TO SAID VOLTAGE DIVIDER; MEANS CONNECTED IN SAID VOLTAGE DIVIDER NEAR THE CATHODE OF SAID BY-PASS RECTIFIER RESPONSIVE TO OPENING OF SAID SHORT-CIRCUITING MEANS TO INCREASE THE VOLTAGE BETWEEN THE CATHODE AND THE ADJACENT INTERMEDIATE ELECTRODE; SAID INCREASED VOLTAGE BEING OF THE SAME ORDER OF MAGNITUDE AS THE VOLTAGE ACROSS AN ARC ARISING UPON OPENING OF SAID SHORT CIRCUIT MEANS; BUT BEING SUBSTANTIALLY LESS THAN THE NORMAL VOLTAGE OVER THE CORRESPONDING CONSTITUENT CONVERTER. 